I. Field of the Invention
The present invention relates generally to satellite communication systems and more particularly to detecting and selecting satellite communication signals using multiple antennas.
II. Description of the Related Art
A variety of multiple access communication systems and techniques have been developed for transferring information among a large number of system users. However, spread spectrum modulation techniques, such as those used in code division multiple access (CDMA) communication systems provide significant advantages over other modulation schemes, for example, especially when providing service for a large number of communication system users. Such techniques are disclosed in the teachings of U.S. Pat. No. 4,901,307, which issued Feb. 13, 1990 under the title Spread Spectrum Multiple Access Communication System Using Satellite or Terrestrial Repeaters, and U.S. patent application Ser. No. 08/368,570, filed under the title Method and Apparatus for Using Full Spectrum Transmitted Power in a Spread Spectrum Communication System for Tracking Individual Recipient Phase Time and Energy, both of which are assigned to the assignee of the present invention and incorporated herein by reference.
The above-mentioned patents disclose multiple access communication systems in which a large number of generally mobile or remote system users each employ at least one transceiver to communicate with other system users or users of other connected systems, such as a public telephone switching network. The transceivers, also referred to as user terminals, can communicate through gateways and satellites, or terrestrial base stations (also sometimes referred to as cell-sites) or other relay terminals.
Base stations cover regions (also sometimes referred to as cells), while satellites have footprints on the surface of the Earth. In either system, capacity gains can be achieved by sectoring, or subdividing, the geographical regions being covered. Cells can be divided into "sectors" by using directional antennas at the base station. Similarly, a satellite's footprint can be divided geographically into "beams," through the use of beam-forming antenna systems. These techniques for subdividing a coverage region can be thought of as creating isolation using relative antenna directionality or space division multiplexing. In addition, provided there is available bandwidth, each of these subdivisions, either sectors or beams, can be channelized. One way to channelize these subdivisions is to assign multiple CDMA channels through the use of frequency division multiplexing (FDM). In satellite systems, each CDMA channel is referred to as a "sub-beam," because there may be several of these per "beam."
In communication systems employing CDMA, separate links are used to transmit communication signals to and from a gateway or base station. A forward link refers to communication signals originating at the gateway or base station and transmitted to a system user. A reverse link refers to communication signals originating at a user terminal and transmitted to the gateway or base station.
In one type of spread-spectrum communication system, one or more preselected pseudo-noise (PN) code sequences are used to modulate or "spread" user information signals over a predetermined spectral band prior to modulation onto a carrier for transmission as communication signals. PN spreading, a method of spread-spectrum transmission that is well known in the art, produces a signal for transmission that has a bandwidth much greater than that of the data signal. In the base station- or gateway-to-user terminal communication link, PN spreading codes or binary sequences are used to discriminate between signals transmitted by different base stations or over different beams. These codes are typically shared by all communication signals within a given cell or sub-beam.
A pair of pseudonoise (PN) code sequences can be used to modulate or "spread" information signals. Typically, one PN code sequence is used to modulate an in-phase (I) channel while the other PN code sequence is used to modulate a quadrature-phase (Q) channel. This PN modulation or encoding occurs before the information signals are modulated by a carrier signal and transmitted as communication signals. The PN spreading codes are also referred to as short PN codes because they are relatively "short" when compared with other PN codes used by a communication system.
A common goal in the design of such multiple-access communications systems is to achieve the highest possible user capacity, that is, to enable the largest possible number of users to access the system simultaneously. System capacity can be limited by several factors, such as the number of user codes and CDMA channels available. However, spread spectrum systems are "power limited", that is by the total amount of power allowed by all users to prevent unacceptable interference, and generally it is the amount of power required to maintain forward link communications to system users that limits system capacity the most. If the amount of power required to maintain just a few of these links is large enough, the total power allocation is consumed well before the number of codes or frequencies available for more users are exhausted.
Therefore, it is generally necessary to minimize the power required to "connect" each user or maintain their forward link in order to leave power for other users and increase system capacity. This can be accomplished by increasing the ratio G/T of antenna gain G to receiver noise temperature T for each user. The higher this ratio or the antenna gain for each user, the less power required by that user for a link. When the gain of each user antenna is high then a sufficiently low amount of power is required to maintain the forward link and power is available for others.
In satellite communication systems, another design goal is to track or acquire and communicate with multiple satellites simultaneously. One reason behind this goal is the desire to improve signal reception using signal diversity. Another reason is to accommodate communications with satellites which are in range, or in view, for relatively short periods of time. A common approach to achieving these goals is to optimize antenna design.
One such design optimization is to utilize large, steerable, highly directional antennas. Another such design optimization is to combine the signals of multiple antennas to form a steerable beam. One disadvantage of these approaches is that the manufacture and integration of such antennas is both complex and expensive.
Another such design optimization is to use multiple antennas, such as "patch" or helical antenna elements, that each cover a different sector of the sky. That is, each antenna has a radiation pattern that is optimized to cover a specific region of the sky or of a satellite constellation orbital pattern. This results in increased directivity and increased gain or G/T per antenna. A significant advantage of this approach is that less expensive antennas can be used. Unfortunately, the primary barrier to the use of multiple inexpensive antennas is the difficulty of detecting and selecting the appropriate signals from the antennas.